A family wit two patients with muscle weakness was referred for studies of familial myositis. The correct diagnosis was discovered to be phophofructokinase deficiency, A single base mutation in an intron leading to a splicing defect and the excision of an exon was discovered in the DNA of the patients and has been traced in the family. The influence of the deletion on the structure and function of the enzyme is being studied. A family with two patients with muscle weakness and respiratory insufficiency was referred for studies of familial myositis. The correct diagnosis was discovered to be acid maltase deficiency. The genetic basis of the defect has been studied. The patient seen at NIH and her father were shown to have a deletion encompassing an entire exon and parts of both surrounding introns. An identical deletion was found in two patients (one Canadian and one Dutch) with the fatal infantile form of the disease. Hence, the identity of the mutations on the other alleles in these patients should reveal the reason that the disease is sometimes expressed in infancy and sometimes in midlife. Two patients with mitochondrial myopathies and one with myoadenylate deaminase deficiency were uncovered in the past year and will be studied in further detail.